Fuel cell systems are typically designed to generate power through a reaction between a hydrogen-rich fuel gas and air within a fuel cell, the fuel gas being supplied as a reducing gas to the anode of the fuel cell whereas air is supplied as an oxidizing gas to the cathode. The fuel gas is generally produced from a raw material gas (such as city gas) and vapor by causing a steam reforming reaction in a fuel treating unit called “reformer”.
While the fuel cell systems are in operation, a portion of the fuel gas supplied to the anode of the fuel cell flows back to the burner as an offgas and is combusted in the burner to be utilized as a heat source for the fuel treating unit. Therefore, the fuel treating unit, fuel cell and various pipes are full of combustible fuel gas.
There have been proposed appropriate post-treatment (purge treatment) techniques for treating the fuel gas that fills up such fuel cell systems when they are shut down. As one example of such techniques, an air purge technique for finally purging the fuel gas by air has been introduced (see e.g., Patent Document 1).
According to Patent Document 1, while power generation in the fuel cell is stopped, the hydrogen-containing fuel gas remaining in the fuel gas passage is evacuated using vapor which has been generated from water supplied from the water feeding device to the fuel treating unit, and then, air is made to flow from a purging air feeding device into the fuel gas passage, thereby finally effecting an air purge. In this fuel cell system, after the hydrogen gas in the fuel cell has been thus eliminated using vapor, air is supplied to the inside of the fuel cell, whereby the corrosion of the passages by moisture beads generated from the vapor can be prevented.
Such an air purge treatment technique can eliminate the need for a nitrogen gas storage tank and is therefore economically effective, compared to the conventional nitrogen gas purge treatment technique according to which upon stopping of the power generation of the fuel cell system, an inert gas (nitrogen gas) flows into the burner by way of the fuel treating unit and the fuel cell, thereby guiding the gas (fuel gas etc.) remaining in the unit and fuel cell to the burner to make it undergo a treatment in the burner.
Patent Document 1: International Publication Gazette No. 01/97312